fix NextClear out of range problem

[liaojie]

when we come to the last uint64 digit in NextClear, maybe we should be aware that not all bits of this one are included.
For example, v:= bitset.New(100), only the first 36 bits of the last uint64 should be available. Otherwise, after

   for i := uint(0); i != 100; i++ {
           v.Set(i)
   }

would be somehow wired to return the 101th bit as an unset bit since the user only created a 100 length bitset.

[lemire]

Looks good to me.

[coveralls]

Coverage increased (+0.5%) to 84.272% when pulling f8f4dd5 on liaojie:master into 5c3c0fc on willf:master.